[ruby-core:105124] [Ruby master Feature#13683] Add strict Enumerable#single

Issue #13683 has been updated by meisel (Michael Eisel).


+1 for this, I've needed it in many cases, generally where I have to select an element from a set and there's no stable/documented way of doing this. So, I find some criteria that seem to work, but would like to protect against my criteria being wrong or invalidated by future changes. In the past, I've needed it for sets like sibling directories and sibling XML nodes.

As for the name, I think `#take_only` would be another reasonable option

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Feature #13683: Add strict Enumerable#single
https://bugs.ruby-lang.org/issues/13683#change-93537

* Author: dnagir (Dmytrii Nagirniak)
* Status: Feedback
* Priority: Normal
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### Summary

This is inspired by other languages and frameworks, such as LINQ's [Single](https://msdn.microsoft.com/en-us/library/bb155325%28v=vs.110%29.aspx) (pardon MSDN reference), which has very big distinction between `first` and `single` element of a
collection.

- `first` normally returns the top element, and the developer assumes
  there could be many;
- `single` returns one and only one element, and it is an error if there
  are none or more than one.

We, in Ruby world, very often write `fetch_by('something').first`
assuming there's only one element that can be returned there.

But in majority of the cases, we really want a `single` element.

The problems with using `first` in this case:

- developer needs to explicitly double check the result isn't `nil`
- in case of corrupted data (more than one item returned), it will never
  be noticed

`Enumerable#single` addresses those problems in a very strong and
specific way that may save the world by simply switching from `first` to
`single`.

### Other information

- we may come with a better internal implementation (than `self.map`)
- better name could be used, maybe `only` is better, or a bang version?
- re-consider the "block" implementation in favour of a separate method (`single!`, `single_or { 'default' }`)


The original implementation is on the ActiveSupport https://github.com/rails/rails/pull/26206
But it was suggested to discuss the possibility of adding it to Ruby which would be amazing.



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